Cautious implementation of low impact development recommended
Today, many communities want the well-publicized benefits of green infrastructure, aka low impact development. However, LID with its many benefits may also have potentially adverse impacts. To strike the right balance, local governments must be aware of the potential risks.
LID Best Management Practices use dispersion and infiltration of stormwater runoff to mimic a natural hydrologic regime. Yet this may not always be compatible with existing or planned development. Possible impacts due to significant changes in interflow (shallow groundwater movement) or deep groundwater recharge include:
- Increased landslide risks
- Down gradient seepage and surface flooding
- Adverse impacts to waterways and wetlands
Although it may be easier to balance these impacts in new developments, caution and consideration must be exercised with any LID BMP implementation.
To address these risks, it is important to understand site-specific constraints. Typically regional codes include infiltration facilities siting criteria: setbacks to landslide areas and wells and design restrictions for shallow groundwater and soil infiltration rates. It is best to apply engineering judgement in addition to these siting criteria. For instance, a typical setback may not be suitable if a substantial increase in infiltration volumes is projected and the area has known landslide risks. Another judgement call might be to limit LID retrofits where significant infiltration could raise shallow groundwater at existing structures.
The most complicated consideration is most likely the interplay between shallow interflow and deep recharge. The amount of retention, evapotranspiration and deep aquifer recharge for an undeveloped land area can be estimated based on the land cover and the local soils. Knowing the land cover and soil type means simplified assumptions can be applied to estimate the volume of interflow. The interflow in non-forested lands is greater than forested lands, as vegetation provides more retention and evapotranspiration.
Conversion of undeveloped land with moderately to highly permeable soils using LID BMPs results in mere small changes in deep recharge. Similarly, low permeability soils also experience small changes when shallow infiltration BMPs are implemented. However, using deep infiltration techniques within low permeability soils significantly increases deep recharge and eliminates interflow.
Within existing development, LID BMP retrofits typically result in significant increases of deep recharge. Further, when shallow infiltration BMPs are used in low permeability soils, an increase in interflow is expected.
What does this all mean?
Municipal leaders need an improved understanding of their specific land use, both historical and current. Leadership must be aware of land forms, such as proximity to landslide prone areas and protected resource areas such as wetlands, waterways and well fields. And finally they need to develop strategic approaches to limiting adverse impacts to their community.
A greater body of knowledge will reduce the risk to a community. It will also improve our guidance when helping you implement LID BMPs.
If you are interested in reading more on this subject, view Low Impact Development & Groundwater on page 13 of The Water Report, an Envirotech Publication.
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